Desorption as a Rate Limiting Step for Gas Permeation through a Polymer Membrane

Abstract

Under certain conditions, permeation of gases through polymer membranes is not adequately represented by classic diffusion theory. Specifically, thin films can present stronger resistance to gas transfer than would be suggested by the material “permeability” alone, especially when one side of the membrane is wetted. It is proposed that kinetics of gas transfer at the surface may explain these discrepancies, and a series of experiments is proposed to quantify these effects. By measuring CO2 permeation through silicone membranes of different thicknesses, estimates of the finite rate constants governing gas exchange at the surface of a nonwetted membrane can be obtained along with the diffusivity of the material. In addition, observations from a “relaxation” experiment for CO2 transfer through a silicone membrane from a gas phase into a dissolved phase allows the rate constants at the wetted surface to be estimated. Estimates of diffusivity of CO2 through silicone from these experiments (1.54 × 10-5 cm2 s-1 at 23 °C) are reasonably close to other values inferred from the literature, and finite positive values for the surface rate constants indicate the importance of surface effects, especially for wetted membranes.